1. Field of the Invention
The present invention relates to a CVD (Chemical Vapor Deposition) method and an apparatus therefor, and more particularly, to an improvement of a CVD method suitable for forming a thin film on a semiconductor wafer and an apparatus therefor.
2. Description of the Background Art
A sectional view of a CVD apparatus disclosed in. Japanese Patent Laying-Open No. 2-283696 is shown in FIG. 1. This CVD apparatus comprises a heating stage for holding a semiconductor wafer 1 upon which a CVD film (not shown) is to be formed; a heater 3 for heating stage 2; a reaction chamber 4 in which semiconductor wafer 1 is to be processed by CVD; a gas head 5 having a plurality of gas blowing openings 6 and provided opposing stage 2 with a distance therebetween for supplying reaction gas A; a reaction chamber sidewall 7 surrounding reaction chamber 4; an exhaust gas ring 8; an exhaust gas chamber 9; an exhaust path 10 provided in the proximity of reaction chamber sidewall 7 for passing exhaust C; an exhaust outlet 11; an exhaust flange 12; a stage clearance 14 for preventing heat dissipation of stage 2 due to conduction and for allowing rotation of stage 2; an N.sub.2 blowing opening 15; and an N.sub.2 blowing ring 16.
A CVD apparatus of the above-described structure can form a CVD film on semiconductor wafer 1 by directing reaction gas A from gas blowing openings 6 towards semiconductor wafer 1 previously heated upon stage 2. The growth rate of the CVD film depends upon the gas concentration supplied from gas head 5 to semiconductor wafer 1. It is therefore necessary to maintain the reaction gas A flow in a stable and uniform manner so that the concentration of reaction gas A is stable and uniform at an arbitrary portion above semiconductor wafer 1 in order to obtain a uniform thickness of a CVD film.
Reaction gas A generates undesirable precipitated particles by CVD reaction which adhere to some portions on the reaction chamber's inside wall. Precipitated particles D afterwards fall off from the inside wall to adhere to semiconductor wafer 1. Precipitated particles D are responsible for decrease in product yield. If the reaction chamber was frequently cleaned for removing precipitated particles D to avoid decrease in yield, the productivity was lowered. There is the need of decreasing generation of precipitated particles and to discharge the generated particles D from reaction chamber 4 along with exhaust C.
The major portion of a continuous type CVD apparatus of prior art is shown in FIG. 2. An enlarged sectional view of the gas head portion of the CVD apparatus of FIG. 2 is shown in FIG. 3. This continuous type CVD apparatus comprises a stage 2 for transporting a plurality of semiconductor wafers 1 on which a CVD film (not shown) is to be formed; a heater 3 for heating stage 2 and semiconductor wafer 1 mounted thereupon to a predetermined temperature; a plurality of gas heads 5 having a plurality of slits all over and opposing semiconductor wafer 1 for supplying reaction gas A in a uniform manner to semiconductor wafer 1; and an exhaust cover 20 provided to cover the plurality of gas heads 5 for gathering exhaust C.
In the conventional continuous type CVD apparatus of the above-described structure, reaction gas A supplied from the end of gas head 5 towards semiconductor wafer 1 spreads out horizontally to flow out as exhaust C without reaching semiconductor wafer 1. Reaction gas A not reaching wafer 1 generates by CVD reaction undesirable precipitated particles D which adhere to the outside wall of gas head 5, the inner sidewall of exhaust cover 20 and the like. These precipitated particles D fall off to adhere to semiconductor wafer 1, resulting in decrease in product yield. It is necessary to periodically stop the apparatus to remove precipitated particles D. This is responsible for reduction of operating efficiency of the apparatus.